Power systems such as electrical power distribution or transmission systems are used to supply, transmit and use electric power. High Voltage Direct Current (HVDC) power transmission is becoming increasingly important due to increasing need for power supply or delivery and interconnected power transmission and distribution systems.
Electro-mechanical oscillations between interconnected power-generating machines, e.g. electrical generators, may occur in power systems. Such oscillations, which for example may be oscillations in machine speed and/or frequency, may be referred to as modal oscillations. Oscillations associated with a single machine or generator are often referred to as local modes, which oscillations generally are localized to the machine or generator and the transmission line connecting it to the rest of the power system. Local modes generally have frequencies in the range of 1-2 Hz. Oscillations associated with groups of machines or generators are often referred to as interarea modes. Interarea modes generally have frequencies in the range of 1 Hz or less, e.g. between 0.1 Hz and 0.8 Hz. Interarea mode oscillations may for example involve two coherent groups of machines or generators which are swinging against each other. Interarea mode oscillations generally involve a great number of parts or portions of the power system and are generally characterized by a highly non-linear behavior. Interarea oscillations may for example be triggered by power system disturbances, such as faults occurring in the power system, switchings, and/or sudden changes in power generation and/or loads in the power system.
In point-to-point (PTP) HVDC power transmissions, two Alternating Current (AC)/Direct Current (DC) converter terminals interconnected by at least one transmission line, e.g., an underground cable and/or an overhead line, are employed. PTP HVDC power transmissions have since the 1980's been used to damp modal oscillations in power systems. Damping or reducing modal oscillations in power systems may be referred to as power oscillation damping (POD). Local modes are usually damped by means of so called power system stabilizers (PSSs), which may provide a sufficient damping of such modes. For interarea mode oscillations, it is in general desired or even required that sufficient damping of such oscillations can be performed in a relatively short amount of time, for example in a few tens of seconds or less. ‘Sufficient’ damping of interarea mode oscillations may for example mean that the oscillations are damped such that their amplitude falls below some threshold value.